The street Lawyer by John Grisham

Michael was in a hurry. He was scrambling up the ladder at Drake & Sweeney, a giant D.C. law firm with eight hundred lawyers. The money was good and getting better; a partnership was three years away. He was a rising star with no time to waste, no time to stop, no time to toss a few coins into the cups of panhandlers. No time for a conscience.

But a violent encounter with a homeless man stopped him cold. Michael survived; his assailant did not. Who was this man? Michael did some digging, and learned that he was a mentally ill veteran who'd been in and out of shelters for many years. Then Michael dug a little deeper, and found a dirty secret, and the secret involved Drake & Sweeney.